Processing gas mixtures



April 1961 F. G. PEARCE ETAL 2,977,771

PROCESS GAS MIXTURES Filed June 1'7, 195'? 2 Sheets-Sheet 1 BY SCOTT W.WALKER FIG. I M 7 A r roan/5r April 4, 1961 F. G. PEARCE EIAL PROCESSGAS MIXTURES Filed June 17, 1957 2 Sheets-Sheet 2 I I l |ZONE 0F FOGFORMATION I +60 I PACKING TEMPERATURE 0 1.; NO RECIRCULATION g AT WARMEND 2 PRODUCT GAS 35 TEMPERATURE RECIRCULATION u.| AT WARM END l- C l Io 20 40 so so DISTANCE (PERCENT) FROM WARM END OF REGENERATOR FIG. 2

INVENTORS FRANK G. PEARCE By SCOTT W. WALKER ATTOF/V Y "United StatesPatent() PROCESSING GAS MIXTURES Frank G. Pearce and Scott W. Walker,Tulsa, Okla., as-

signors to Pan American Petroleum Corporation,.Tulsa, Okla, acorporation of Delaware V Filed June 17, 1957, Ser. No. 665,916 9Claims. (Cl. 62-13 and subsequent fractionation thereof by firstcompressing I the gas, thereafter cooling it, usually by heat exchangewith a cold product gas stream, liquefying a portion of the cooled gasthrough further heat exchange, and expanding another portion of thecooled compressed gas with refrigeration to the fractionation system.

Cooling of the air feed stream is generally effected by flowing thelatter through one of two sets of regenerators filled with metal packingand which operate in periodically the performance of work therebyfurnishing the necessary reversing cycles between the incoming air feedand the I outgoing cold product gas streams. In cooling the air feedstream in this manner, however, water which is carried into the systemas vapor is condensed and deposited as liquid at the warm end of theregenerators, i.e., about thelast one-eighth to one-fifth of the lengthof the regenerator. The air flow cycle is continued generally for abouttwo or three minutes at elevated pressures, usually 60 to 100 p.s.i.g.Thereafter, the set of regenerators through which air was flowing isd'epressurized against the other group of regenerators through which acold product gas stream has beenflowing. The latter group ofregenerators ordinarily operates at about 8 to 10 p.s.i.g. Since thisdecrease in pressure occurs very rapidly, an appreciable quantity of theliquid water present in the warm end of the regenerators in which airflow has just been discontinued, is discharged therefrom in the form ofa mist or fog. The quantity of water eliminated from the regenerators inthis manner, is a substantial amount, i.e., in the range of about 20 to50 percent of the water introduced into the system as vapor. The liquidwater presentin the product gas stream is considered to be the directresult of fog formation caused by vaporization of water from the packingin the warm end of the regenerators into the colder gas stream.

As an alternate to the above mentioned regenerative type coldexchangers, countercurrent reversing cold exchangers may be employed.These permit a simultaneous and efficient heat interchange betweenpassageways containing countercurrently flowing streams of the gasmixture to be separated and returning cold products. Ex-

changers of this type comprise essentially a plurality of parallel pathsfor the stream in each passageway. These paths are so metal bondedtogether as to establish a metal-to-metal thermal contact throughout theentire contact length of the unit. Likewise the individual passagewaysof an exchanger are joined with metal-to-metal contact. Reversing coldexchangers of this type, therefore, are characterized by possessing ahigh rate of heat transfer and a thermal efficiency unaffected by cycletime, since little dependence is placed on storage of heat in metal.

'per day would be as much as 570,000 B.t.u.s/hr.

2,977,771 Patented Apr. 4, 1961 These'reversing cold exchangers are alsoutilized to remove all of the higher boiling impurities in gaseousmixtures such as air, particularly for separations conducted atrelatively low pressures, such removal being accomplished byperiodically alternating the flow of warm incoming feed gas and abackward returning cold product stream between at least two passagewaysof the exchanger. Thus, during one-half of the reversing cycle when air,for example, is being cooled, water and other condensible impurities aredeposited therefrom and accumulate on the metal surfaces of thepassageway through which the air at that time is flowing. Then, beforethe accumulation has become great enough to interfere with the operationof that particular passageway the countercurrently flowing streams areinterchanged to enable the backward returning anhydrous product to flowover the accumulated deposits and re-evaporate them.

The loss in exchanger refrigeration, mentioned above, could be avoidedif the total amount of water introduced as vapor were eliminated fromthe system as vapor. However, if a part of the water entering as a vaporis expelled as a liquidas would be the case where a water fog isphysically entrained with the produce gas streamthe heat of condensationconstitutes a direct heat input to the system. In tonnage oxygen plants,for example, where this problem becomes most acute, the refrigerationlosses of a plant having a capacity of about 1,000 tons of oxygen Whenit'is realized that the design heat leak for a plant of this size isslightly less than percent of this figure, the significance of such anenthalpy input becomes apparent.

Accordingly, it isan object of our invention to prevent substantialrefrigeration losses from the system, such losses being caused byentrainment with the cold product gas of appreciable quantities ofliquid water and/or similar substances deposited in the warm end of theexchanger during the air flow cycle. Briefly, this object isaccomplished by providing means for decreasing the difference intemperature between said warm end and the cold product gas present insaid warm end during the refrigeration cycle.

In accordance with our invention, we are able to avoid refrigerationlosses of the above type by taking a portion of the product streamissuing from the warm end of the regenerator or reversing exchanger, andreturning such portion to the regenerator or exchanger at about thelevel therein farthest removed from said warm end where liquid waterexists and where substantial quantities of ice begin to form.

The level at which the warm product (50 to 60 F.) stream is recirculatedto the unit ordinarily is located at a distance from the warm endthereof corresponding to from about 10 to about 20 percent of the lengthof the unit. Under such conditions the warm recirculated stream raisesthe temperature of the metal packing in the regenerator, or of the metalwalls of the reversing exchanger, from the point of injection to theopposite end of the warm portion of the unit. This, in turn, decreasesthe difference in temperature between the cold product stream flowingthrough the unit and that portion of the packing or wall surface onwhich liquid water is ordinarily deposited.

As previously pointed out, the amount of water discharged as liquid fromthe warm end of the unit, together with the product gas, amount to fromabout 20 to about 50 percent of the amount introduced into the systemvia the air feed stream. Accordingly, in order to prevent the aforesaidquantity of water from being rejected from the system as liquid alongwith the product gas, from about 10 to about 30 percent of the warmproduct stream ordinarily should be recirculated as contemplated herein.The actual amount of warm product recirculated in accordance with ourinvention, however, will depend upon the performance characteristics ofthe regenerators or reversing exchangers used. In any event, thequantity of warm product gas recirculated will-correspond approximatelyto the amount required to bring the temperature of the packing, forexample, in the case of a regenerative type unit, up to a level suchthat the quantity of liquid water remaining in the warm end is held to aminimum during the refrigeration cycle.

For a better understanding of our invention, reference is made to theaccompanying drawings in which:

Figure 1 represents a diagrammatic form of apparatus included within thescope of our invention, and

Figure 2 is a plot showing the temperature profile in said apparatuswith and without the application of our invention.

The flow rates, as well as other quantities appearing in the descriptionof the drawings, are given on the basis of an operation capable ofproducing 1,000 tonsof oxygen per day. Referring now to Figure 1, thereisshown a single regenerator 2 filled with aluminum packing 4. The upperend of the regenerator is connected to a flow line 6, which in turncommunicates directly with common header 8 and indirectly on alternatecycles with air feed header 10 and product gas header 12. At the lowerend of regenerator 2,-a flow line 14 in similar fashion communicateswith common header 16, cold product header 18 and cold air header 20. Itwill be understood, of course, that in actual practice, other generatorssuch as 2, are similarly connected to headers S, 10, 12, 16, 18 and 20.

Clean air is introduced at about 100 p.s.i.g. and 60 F. into line 22 atthe rate of 5,700 M s.c.f.h. This compressed air stream then flowsthrough air feed header 10, valved line 24, common header 8, line 6 andinto regenerator 2. The air is rapidly cooled by coming into contactwith cold aluminum packing 4. At the warm end of regenerator 2,generally designated at 26, water vapor condenses and is deposited onthe packing. The water condensed from the air feed stream and thusdeposited on the packing amounts to about 500 lbs./ hour. After the airhas passed the warm end of the regenerator, it is essentially anhydrousand becomes progressively colder until it is withdrawn from the base ofregenerator 2 at a temperature of about 265 F. through line 14. Fromthis point the cold air stream is taken through common header 16, valvedline 28, cold air header and out through line 30. The stream in the lineis then subsequently split with a major port-ion going to a suitablehigh pressure fractionating tower and the remainder passing through anexpander, after which the resulting cold vapors are fed to anappropriate low pressure fractionating tower to furnish the requiredrefrigeration to the system. Since the processing of the air streamafter it reaches line 30 forms no part of our invention and inasmuch asmethods of processing this stream are well known to the art, no furtherdiscussion of the procedure and equipment used for this purpose isconsidered necessary.

After the required air flow cycle through regenerator 2, which may be aperiod of two or three minutes, valves 32 and 34 are closed and valve 36is opened. Operation of these valves is controlled by automatic timingdevice 38 actuating line 40 which in turn is operatively connected tovalves 32, 34 and 36. A returning cold product nitrogen or oxygen streamat a temperature of about 275 F. then surges rapidly at a pressure ofabout 8 to 10 p.s.i.g. through line 42, cold product header 18, line 44and check valve 46, common header 16 and into regenerator 2 via line 14.As the cold product gas rises through the regenerator it cools offpacking 4 so that the latter will be suitable for use in the next airflow cycle.

This gas as it flows upwardly through the re- 7 generator is not onlycold but is extremely dry. As the 4 product gas approaches the warm endof regenerator 2, it is at a temperature of about 40 F. Water that hasbeen deposited on packing 4 during the air flow cycle is alreadypartially vaporized and the gas phase in the warm end of regenerator 2,is saturated with water vapor. However, as the colder (40 F.) productgas stream contacts the vater'vapor in the warm end of the regenerator,conditions are produced which favor fog formation and accordingly, anappreciable portion, e.g., 20 to 50 percent of this vapor is convertedto a fog or mist of liquid water droplets. In conventional procedures itis apparent that a net loss in refrigeration from the regenerators willbe incurred if the water vapor leaving the system together with theproduct gas in line 6 is less than thewater vapor brought in via the airfeed system. The amount of refrigeration lost in this manner, therefore,corresponds to the evaporative cooling that would have been furnishedthis system if such liquid water has been converted to vapor in theregenerator prior to removal therefrom. In accordance with ourinvention, however, conditions favoring fog formation are minimized bybleeding off between about 10 to about 30 percent of this product gasfrom line 6 through line 50 and forcing this warm stream (at about 60F.) back into regenerator 2 at the level indicated by means of blower52.

By inj-cting the warmer product gas back into the regenerator in thismanner, it will be seen that the temperature of the cold product gasentering the warm end of regenerator 2 is increased, thereby decreasingthe difference in temperature between said product gas and aluminumpacking 4, which in turn renders conditions for fog formation lessfavorable. For any given flow conditions, regenerator design, etc., thequantity and rate of warm product gas recirculated through line 50 toregenerator 2 can readily be determined and regulated by means of valve54, once the process is properly lined out. Thus, in applying theprinciples of our invention to a conventional oxygen plant, the amountof warm product gas injected into regenerator 2 via line 50 during agiven cycle, should gradually be increased until the Water vapor contentof the product stream issuing through line 48, is substantially equal tothe amount of water vapor in air feed line 22.

The effect of recirculating warm product gas to the warm end of theregenerator, as described above, on the difference in temperature of themetal packing and of the product gas travelling toward said warm end, isclearly shown in Figure 2. The improvement afforded, i.e., decrease intemperature difference between the packing and product gas at the warmend of the regenerator, by our invention is evident from a comparison ofthe packing temperature profile (curve A) with broken lined curve'B(obtained by warm product gas recirculation). The substantial differencein temperature existing at the warm end between the packing (curve A)and the product gas temperature when no warm product gas is recirculated(curve C) is likewise quite apparent. From these curves it is obviousthat the opportunity for fog formation has been materially decreased byrecirculation of warm product gas in accordance with our invention. Thearea of the regenerator where this result is particularly noticed isthat portion of Figure 2 designated zone of fog formation.

At the flow rates given above, the increase in refrigeration efficiencyobtained in regenerator performance, when employing the process of ourinvention, over regenerator efficiency secured without such improvement,but under otherwise identical conditions, amounts to from approximately5 to about 15 percent, measured in terms of oxygen production.

From the foregoing description it will be apparent that our inventionhas many possible applications in gasseparation and related'fields. Itis particularly applicable to processes for separation of gasescontaining vapors condensible under the conditions of operation andwhich ordinarily remain liquid in the warm end of the exchanger duringthe refrigeration cycle.

In the present description and claims the expression warm end used withreference to that portion of the regenerator or other reversing heatexchange zones from whichproduct gas is recovered, is intended todesignate that part of the regenerator or reversing heat exchange zonein which liquid water is present. Also it is to be pointed out that inconstruing the scope of the claims which follow, the expression pair ofregenerative cooling paths is intended to mean either two individualregenerators or two individual groups of such regenerators. For thepurpose of this description, regenerative and reversing exchangers areto be considered equivalents.

We claim:

1. In a process for recovering a gaseous component from a gaseousmixture containing water vapors wherein a compressed stream of saidgaseous mixture is cooled by passage of said stream through a first heatexchange path progressively decreasing in temperature from end to end,whereby said vapors are condensed and deposited as liquid water inthewarm end of said first path while' an outflowing cold product streamis simultaneously oounterflowed through a second heat exchange pathprogressively increasing in temperature from end to end, withdrawingsaid product steam from the warm end of said second path, periodicallyinterchanging the flow of said compressed stream and said product streamin said paths so that each path undergoes alternate charging andrefrigeration cycles, the method of improving the refrigeration capacityof said paths which comprises recirculating to a point upstream fromwhich said product stream was withdrawn a portion of said withdrawnproduct stream to the warm end of said second path and in a portion of.said second path where liquid water exists while the latter isundergoing said refrigeration cycle whereby the temperature differenceis decreased between the warm end in said second path and said productstream 7 in the warm end of said second path, the amount ofsaid productstream circulated being sufficient to render the' amount of said vaporsin said withdrawn product stream about equal tovthe amount of saidvapors in said compressed stream. 2. The. process ofclaim l wherein saidrecirculated portion is introduced into the warm end of said second pathat a distance from the outlet of said warm end corresponding to not morethan about 20 percent of the length of said path.

3. The process ofclaim 1 in which the amount of product streamrecirculated corresponds from about 10 counter-flowed through the otherof said paths thereby refrigerating said other path, withdrawing saidproduct steam from the warm end of said other path, and wherein the flowin said paths is periodically reversed so that each path undergoesalternate charging and refrigeration cycles, the improvement whichcomprises recirculating to a point upstream from which said productstream was withdrawn a portion of said product stream thus withdrawn tothe Warm end of said other of said paths and in a portion of said otherof said paths where liquid Water exists while said other of said pathsis undergoing said refrigeration cycle whereby the temperaturedifference is decreased between the warm end of said other path and saidproduct stream in the warm end of said other path, the amount of saidproduct stream recirculated being suflicient to render the amount ofsaid vapors in said withdrawn product stream about equal to the amountend of said other path.

7. The process of claim 5 wherein said recirculated portion isintroduced into the warm end of said other of said paths at a distancefrom the outlet of said warm end corresponding to not more than about 20percent of I .the length of said other of said paths.

' 8. In a process for recovering a gaseous component from a gaseousmixture containing water vapors wherein a condensible stream of saidgaseous mixture is passed in one direction of fiow through a reversingheat exchange zone in indirect heat exchange relation with acounterflowing cold product stream along a path therein progressivelydecreasing in temperature from end to end to cool said stream therebyconverting said vapors into liquid water and depositing the latter inthe warm end of said path, wherein a cold product stream is passedsubsequently to about 30 percent of that withdrawn from the warm end ofthe heat exchange path undergoing the refrigeration cycle.

4. The process of claim 3 wherein the recirculated product stream isreturned to the heat exchange path undergoing said refrigeration cycleat about the lowermost level in such path where saidli'quid water ispresent.

5. In a process for recovering'a' gaseous component from agaseousmixture containing water vapors wherein a compressed stream ofsaid gaseous mixture is cooled by passage through one of a pair ofregenerative cooling paths and said vapors are thereby converted toliquid water and'deposited in the warm end of said one of said pathswhile outflowing cold product gas is simultaneously vthrough the samepath in the opposite direction of flow after said compressed stream hasceased fiow therein, and

withdrawing said product stream from the warm end of said path, themethod forimproving the refrigeration efficiency of said path whichcomprises recirculating to a point upstream from which said productstream was withdrawn a portion of said product stream thus withdrawn-to'the warm end of said path and in a portion of said path where liquidwater exists 'while additional quantities of said product streamcontinue to flow through said path whereby the temperature ditierence isdecreased between said warm end and said product stream in said warmend, the amount of said product stream circulated being sufficient torender the amount of said vapors in said withdrawn product stream aboutequal to the amount of said vapors in said compressed stream.

9. The process of claim 8 in which the vapors of the condensible liquidare water vapors and the amount of product stream recirculatedcorresponds to from about 10 to about 30 percent of that withdrawn fromthe warm end of said path.

References Cited in the file of this patent UNITED STATES PATENTS Palmeret al. July 10, 1956

